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An aircraft at FL360 is required to descend to FL120. The aircraft should reach FL120 at 40 NM from the next waypoint. The rate of descent is 2000 ft/min. The average GS is 420 kt. What is the minimum distance from the next waypoint at which descent should start?
  • A
    236 NM
  • B
    166 NM
  • C
    88 NM
  • D
    124 NM

The aircraft is currently at 36 000 ft and is required to descend to 12 000 ft which means that it must descend 24 000 ft.

With a rate of descent of 2 000 ft/min, the aircraft will take 12 minutes to descend 24 000 ft (24 000 ft ÷ 2 000 ft/min).

We must now calculate the distance the aircraft will cover in 12 minutes when flying at a GS of 420 kt: (420 kt x 12 min) ÷ 60 min = 84 NM

Since the aircraft should reach FL120 at 40 NM from the next waypoint, it should start descent at a minimum distance of 124 NM (84 NM + 40 NM).

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